Reverse osmosis (RO) membranes are widely used as permselective membranes in the field of desalination of sea water and salt water, production of industrial water and ultrapure water, waste water recovery, and the like. RO membrane treatment has the advantage of being highly capable of removing ions and low molecular weight organic matter but, on the other hand, requires a higher operating pressure than microfiltration (MF) membranes and ultrafiltration (UF) membranes. In order to increase the water permeability of RO membranes, for example, in polyamide RO membranes, efforts have been made such as controlling the pleated structure of a skin layer to increase the surface area.
In recent years, aquaporin, which is a membrane protein that selectively transports water molecules, attracts attention as a water channel substance, and it is suggested that a phospholipid bilayer membrane incorporating this protein may have a theoretically greater water permeability than conventional polyamide RO membranes (Non Patent Literature 1).
Examples of methods for producing a permselective membrane comprising a phospholipid bilayer membrane that incorporates a water channel substance include a method involving sandwiching between porous supports a lipid bilayer that incorporates a water channel substance, a method involving incorporating a lipid bilayer inside the pores of a porous support, a method involving forming a lipid bilayer around a hydrophobic membrane, and the like (Patent Literature 1).
The method involving sandwiching a phospholipid bilayer membrane between porous supports increases the pressure resistance of the phospholipid bilayer membrane but has problems in that, for example, the porous supports themselves that are brought into contact with water to be treated may be contaminated, concentration polarization may occur in the porous supports and greatly deteriorate the rejection, and the porous supports may serve as resistance and deteriorate water permeability.
Aquaporin A/S of Denmark manufactures RD membranes incorporating aquaporin in a polymer matrix, but the water permeability is only about 1.2 times the water permeability of existing RO membranes, and Non Patent Literature 2 fails to provide a clear description.
In Patent Literature 2, a cationic phospholipid is used to securely support a channel substance on a nanofiltration (NF) membrane, but there is a problem in that it is difficult to cause the intrinsic water permeability of the channel substance to be exerted because the resistance of the NF membrane itself is large.
Non Patent Literature 3 reports the results of supporting phospholipid on a NF membrane to introduce channel-substance amphotericin B, and the water permeation rate is 0.3 L/(m2·h·atm) or less. With the water permeation rate at a 0 to 1 L/(m2·h·atm) level, because ergosterol itself that is caused to be concomitantly present with amphotericin B also has a water permeability increasing effect, there is a problem in that optimum amounts of amphotericin B and ergosterol to be added are not clear.
Concerning these substances it is known that amphotericin B does not form channels when ergosterol is not concomitantly present. When channel formation is evaluated based on the circular dichroism (CD) spectrum of a phospholipid vesicle dispersion, a positive peak appears at 330 nm with amphotericin B alone and, in the case where ergosterol is present and channels are formed, a positive peak appears at 340 to 350 nm, and a negative peak appears at 370 nm and 390 nm (Non Patent Literatures 4 and 5).
However, there is also a report that a CD spectrum obtained in the absence of ergosterol shows peaks detected in similar regions (Non Patent Literature 6), and thus the optimum relationship between amphotericin B and ergosterol necessary for preparing a RO membrane or a forward osmosis (FO) membrane is not known.